EDIT:
Something stupid that I do in shooting games is when a person that is on my team comes around a corner really quick (and I'm expecting an enemy) I shoot them in the head.
Another thing that I did in a shooting game (I think it was COD) is to try to throw a grenade into a building through a window however the grenade hits the wall and rebounds. It went straight behind me into a vehicle were my team mates were hiding behind... I don't have to explain the rest do I.

Trying to defeat the final boss in Kingdom Hearts 2 when i never used any effort to level up throughout the whole game.

You poor guy...
KH2 had the easiest final boss though (imo).

Anyway back on topic another thing I forgot to mention, is in srb2 when I accidentally helped the opposite team win a CTF match. I was completely used and oblivious of what team I was on, probably because I never died and I was hardly near the my base (so I did not know that my true team's rings were mine).

I built a giant sand house with a wooden roof (It was actually quite nice) on Minecraft, on top of a floating sand platform in the air. Y'know, that weird thing where sand stays in the air? Well, once I finished, I removed a single block from the floor and the entire thing collapsed to below, the roof was left up there. All of the blocks went down a big trench, half of them landing in lava. I was so pissed at that. 8D

EDIT: To add insult to injury, I didn't save at all. ._.

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Hai! I use rail rings to prove how cheap and easy they are to use!FUCK RAILS

I don't think so. In my opinion it's the hardest because of that ONE STUPID MOVE where he takes Sora and you have to save him using Riku and he has a clone that just keeps doing this stupid move that makes a wall appear where if you make ONE mistake you'll just constantly get hit by it and u can't have any chance of landing because it hits you really hard so you have to wait until he's done doing that and he does it like 5 or 6 times, taking away so much life because Riku has such a small health bar. I'm lucky i managed to finish that boss recently.

I don't think so. In my opinion it's the hardest because of that ONE STUPID MOVE where he takes Sora and you have to save him using Riku and he has a clone that just keeps doing this stupid move that makes a wall appear where if you make ONE mistake you'll just constantly get hit by it and u can't have any chance of landing because it hits you really hard so you have to wait until he's done doing that and he does it like 5 or 6 times, taking away so much life because Riku has such a small health bar. I'm lucky i managed to finish that boss recently.

Oh that bit, I easily found away to counter Xemnas' clone with Riku. I fire then attack, then I run like hell.

The most stupidest thing I've done was speed running Super Mario 64, making it with a great time till the damn Hazy Maze Cave. There a hidden star in the level, where you have to wall jump to get to it. I completely forgot where it was, ruining my 100% speed run. Forcing me to restart all over again.